who was polonius? why did he hide behind the curtains?

Polonius

The father of Ophelia and Laertes and the chief adviser to the throne of Denmark. Polonius is a windy, pedantic, interfering, suspicious, silly old man, a "rash, intruding fool," in Hamlet's phrase. Polonius is forever fomenting intrigue and hiding behind tapestries to spy. He hatches the theory that Ophelia caused Hamlet to go mad by rejecting him. Polonius' demise is fitting to his flaws. Hamlet accidentally kills the old man while he eavesdrops behind an arras in Gertrude's bedroom. Polonius' death causes his daughter to go mad.

Polonius his behind the curtains to spy on the interaction between his daughter Ophelia and Hamlet, and to see if he and Claudius might determine the cause of Hamlet's irrational behavior and melancholy.